Permanent magnets made substantially of R-TM-B are widely used because of inexpensiveness and high magnetic properties. Because R-TM-B materials have high mechanical strength with little brittleness in addition to excellent magnetic properties, they are less subject to cracking, etc. even when large internal stress is generated by sintering shrinkage. Accordingly, they are suitable for ring magnets having radial anisotropy or multi-polar-anisotropic orientation, largely contributing to providing motors with higher power and smaller sizes.
Because polar-anisotropic ring magnets have surface magnetic flux density waves having higher peaks and closer to a sinusoidal wave after magnetization than those of radial-anisotropic magnets, the polar-anisotropic ring magnets are used as rotors to provide motors with small cogging torque. However, because the polar-anisotropic ring magnets have different orientation directions from portion to portion, cracking called “orientation cracking” occurs easily during sintering. Particularly in the case of large ring magnets, green bodies are likely damaged in production processes, resulting in high risks of cracking.
Instead of using a ring-shaped magnet, a rotor is generally formed by attaching arcuate magnets to a cylindrical yoke. For example, JP 2005-286081 A discloses a method for producing an arcuate magnet having a radial orientation suitable for rotors. However, because arcuate magnets having radial orientation have surface magnetic flux density waves in a trapezoidal form, they cannot be used for rotors needing a sinusoidal waveform. Accordingly, the development of new technologies for producing arcuate magnets having polar-anisotropic orientation has been desired.
JP 2003-199274 A discloses a rotor having a low cogging torque, which comprises arcuate magnets having polar-anisotropic orientation. However, JP 2003-199274 A does not specifically describe a method for producing an arcuate magnet having polar-anisotropic orientation.
A ring magnet having polar-anisotropic orientation can be produced by using, for example, a die apparatus 300 shown in FIG. 10 (corresponding to FIG. 3 of JP 2003-17309 A), which comprises a cavity 330 defined by a core 320 and a die 340 with a spacer 310 on the inner surface, magnetic powder charged into the cavity 330 being oriented to have multi-pole orientation by a magnetic field generated from coils 360 disposed in grooves 350 on the inner surface of the die apparatus 340, to which pulse current is applied. A polar-anisotropic ring magnet produced by such method has a surface magnetic flux density distribution in a circumferential direction, which is close to a sinusoidal waveform, with radial orientation at magnetic poles and circumferential orientation between adjacent magnetic poles (see, for example, JP 2005-44820 A).
To provide an arcuate magnet with such polar-anisotropic orientation, the arcuate magnet should be oriented perpendicularly at circumferential end surfaces, and radially at a circumferential center of its outer arcuate surface, so that a ring magnet obtained by assembling them can have a waveform closer to a sinusoidal wave.
A ring magnet having polar-anisotropic orientation can be molded in a pulse magnetic field generated from coils arranged at even intervals corresponding to the number of magnetic poles as described above. In the case of arcuate magnets having polar-anisotropic orientation, however, it is difficult to adjust the arrangement of magnetic-field-generating coils and voltage applied thereto in a die apparatus having such structure, resulting in difficulty in obtaining ideal arcuate magnets having polar-anisotropic orientation. Accordingly, as in the case of molding block-shaped magnets, a magnetic body should be properly arranged in a parallel magnetic field with its direction changed, to produce an arcuate magnet having polar-anisotropic orientation.
JP 2005-287181 A discloses an arcuate magnet having orientation converged at a center on the outer arcuate side, describing that it provides a rotor with reduced cogging torque. However, because the arcuate magnet described in JP 2005-287181 A has orientation different from ideal polar-anisotropic orientation, the assembling of pluralities of the arcuate magnets in a ring shape would not provide a ring magnet having polar-anisotropic orientation, leaving room for improvement in the reduction of cogging torque.
JP 2002-134314 A discloses a method for producing an arcuate magnet having an arcuate cross section, easy-magnetization axes of magnetic powder in the cross section being converged form the outer surface and both end surfaces toward a center region of the inner surface in projected curves. However, an arcuate magnet produced by the method described in JP 2002-134314 A has a functioning surface on the inner surface, not on the outer surface.
When rotors with large magnets having polar-anisotropic orientation are produced, there is now only a method of assembling parallel-oriented magnet segments in a ring shape having polar-anisotropic orientation. Thus, it is desired to develop a method for producing a sintered arcuate R-TM-B magnet having polar-anisotropic orientation.